Quantum wells (QWs) consist of ultrathin semiconductor layers (i.e., semiconductor wells) sandwiched between barrier layers which have special properties due to the quantum confinement of charge carriers. These layers, in which both electrons and holes are confined, are typically very thin (10 nm or less) and of high quality to confine excitons (electrons and holes), as well as to avoid unwanted recombination sites. For these reasons, the formation of QWs has been realized by high-precision epitaxial growth techniques such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Although such techniques have been widely and successfully used to realize many modem electronic devices, one significant restriction to the choice of material combination is that the lattice constants of the materials for the “walls” and “barriers” should be identical or at least very similar. If two materials have significantly different lattice constants, a continuous single crystal structure cannot be grown without introducing a large number of defects. Therefore, only a few sets of the material combinations are applicable to form QW structures such as Si/SiGe and AlGaAs/GaAs. From the aspect of practical applications, creating QWs on Si substrates implies direct integration of optoelectronics and photonics with Si Complementary Metal-Oxide-Semiconductor (CMOS). Furthermore, deep QWs with a large number of quantized energy levels allow room temperature (RT) operation of on-Si optoelectronic components. However, the QW structures epitaxially grown suffer from limited well depth, and thus a limited number of quantized energy levels due to a relatively small band offset (i.e., available barrier height).
Semiconductor-oxide structured QWs can offer a larger band offset (i.e., larger barrier height) than the aforementioned conventional (from epitaxy) QW structures. However, few such structures have been realized due to the difficulty in forming single crystalline films on top of amorphous oxide materials. Most of the semiconductor-oxide QWs rely on epitaxial growth techniques with special requirements with regard to crystal orientations, and single crystalline-amorphous oxide structures could not be made using these techniques. Wafer bonding has been used as an alternative method to create semiconductor-oxide QWs, but it could only demonstrate a single QW structure due to a process limitation. (See, e.g., Ishikawa, Y., et al., Negative differential conductance due to resonant tunnelling through Sift/single-crystalline-Si double barrier structure. IET Electron. Lett. 37, 1200-1201 (2001).) Moreover, a QW structure that was formed by the wafer bonding process does not provide the desired well/barrier sharpness of energy due to a slow atomic transition between a semiconductor and an oxide by a poor passivation associated with a native oxide. (See, e.g., Lee, T. H., et al., Fabrication Process for Double Barrier Si-Based Quantum Well Resonant Tunneling Diodes (RTD) by UHV Wafer Bonding. ECS Trans. 16(8), 525-530 (2008).)